1. Field of the Invention
This invention relates to a novel cationic polymer emulsion with improved performance, stability and economics. More particularly, the invention relates to a cationic emulsion as produced by emulsion polymerization of an ethylenically unsaturated monomer in the presence of a modified polyvinyl alcohol (hereinafter abbreviated as PVA) containing cationic groups.
2. Description of the Prior Art
Heretofore, in the production of a vinyl polymer emulsion, it has been common practice to employ, as an emulsion stabilizer, a surfactant such as a nonionic emulsifier or an anionic emulsifier and a water-soluble macromolecular substance such as PVA or hydroxyethylcellulose, either singly or in combination. The resultant emulsion particles are of necessity electrically neutral or negatively charged. Emulsions of positively charged particles, i.e., cationic emulsions, are also known and while cationic emulsions have been found to have superior fixation and bonding characteristics, they have so far been used only in special and limited applications. Probably this has been occasioned by the fact that there has been no effective process for the production of a cationic emulsion having satisfactory stability, performance and/or toxicity.
It is already known that a cationic emulsion can be produced by emulsion polymerization using a cationic surfactant as an emulsifying agent. Cationic chloroprene latex and some other products have been obtained by using a laurylamine salt, octadecylamine salt, laurylpyridinium chloride, octylbenzyltrimethylammonium chloride or the like as an emulsifying agent (Japanese Published Unexamined Patent No. 62881/1973). It is also within the knowledge of those skilled in the art that after a vinyl polymer emulsion has been produced using a nonionic or anionic emulsifier, a cationic surfactant or a cation-type surfactant such as alkylaminopolyoxyethylene can be added to the emulsion to control the pH thereof and to thereby render the emulsion particles cationic (U.S. Pat. No. 3,660,324). However, the use of cationic surfactants is subject to stringent regulatory control because of their toxicity. It is also known to make emulsion particles cationic by means of a cation-type polymerization initiator such as 2-azobis(2-methylpropamidinium)dihydrochloride or 2-azobis(2'-isopropylimidazolinium)dihydrochloride or the like (Japanese Published Unexamined Patent No. 92488/1973). However, this method generally provides emulsions having inadequate cationic properties. There has also been proposed an "emulsifier-free emulsion polymerization method" wherein a monomer containing a quaternary cation group is copolymerized with a different monomer in a first stage of polymerization and, then, an emulsion polymerization is conducted in the presence of the resulting copolymer which functions as a protective colloid (U.S. Pat. No. 3,198,762). This method, however, requires sophisticated reaction control and does not easily provide a stable emulsion. It has also been proposed to produce a cationic emulsion by conducting an emulsion polymerization in the presence of cationic starch, cationic cellulose or cationic polyamide-polyamine epichlorohydrin as a protective colloid. Since these water-soluble polymers are low in emulsion stabilizing action and, when used alone, do not provide a stable emulsion, they are generally used in combination with a nonionic surfactant (Japanese Published Unexamined Patent No. 101996/1979, Japanese Publication No. 22922/1971). However, the resulting emulsions are not fully satisfactory with respect to their balance of cationic properties and emulsion stability.
PVA has heretofore been known as a water-soluble polymer which is an excellent protective colloid and has been used in the emulsion polymerization of many ethylenically unsaturated monomers. In vinyl acetate emulsions, especially, or copolymer emulsions based on vinyl acetate and other monomers such as ethylene, acrylic acid esters, maleic acid diesters, vinyl versatate, etc., PVA exhibits an excellent emulsion stabilizing action and permits production of stable emulsions without the aid of surfactants or other emulsifiers. While PVA is thus known as an excellent emulsion stabilizer, it has never been recognized that a cation-modified PVA is useful as an emulsion stabilizer. This is because the several methods proposed for introducing cationic groups into PVA involve high cost and commercially difficult procedures. Moreover, it appears that since many of the methods which have been suggested require modifying PVA with cations under strongly alkaline or strongly acidic conditions, it has been practically impossible to obtain a modified PVA containing vinyl acetate units, that is a partially saponified modified PVA. The emulsion stabilizing action of a completely saponified PVA is generally poor while a partial hydrolysate with an adequate degree of saponification and a suitable distribution of acetyl groups is a good emulsion stabilizer. In view of the industrial utility of cation-modified PVA, the present inventors investigated the possibility of producing such a cation-modified PVA advantageously on a commercial scale. As a result, the present inventors already disclosed such a method in Japanese Patent Application 89078/1979. This production method was an important advance since it permitted a liberal control of the amount of cationic groups and a free control of the degree of saponification. Following the above development, the present inventors performed emulsion-polymerizations of various ethylenically unsaturated monomers using as emulsion stabilizers a variety of cationic group-modified PVA's as synthesized by the above method. Various suitable combinations of cationic group content, degree of saponification and degree of polymerization were tried, and ultimately highly stable cationic emulsions were successfully produced. In producing these excellent cationic emulsions, the conventional emulsion-polymerization procedure using ordinary PVA as an emulsion stabilizer can be utilized without modification and no special operation or special additive is necessary. PVA is simply replaced with a cation-modified PVA. Not only is the method convenient and expedient, but it was found to yield very stable cationic emulsions. Therefore, the method is advantageous in terms of cost and ease of production. Furthermore, in terms of performance, too, these emulsions proved to be superior. They were found to have by far more highly cationic properties than the cationic emulsions of the prior art and commercial cationic emulsions, and accordingly exhibited an improved fixation affinity for pulp and an improved bonding affinity for glass, sand, concrete and other substrates. It was thus confirmed that these cationic emulsions are very useful in several application such as paper-making, civil engineering and construction. The above findings were followed by further research which has resulted in the development of this invention.